The invention relates to a mercury short-arc lamp in accordance with the preamble of Claim 1.
Such lamps are used, for example, as light source for exposure systems for producing semiconductor components, liquid crystal displays (LCD) or printed circuit boards (PCB). The power consumption of these lamps is typically in the region of approximately 1 kW and 10 kW or more.
Arranged inside the discharge vessel of the lamp are two electrodes situated opposite one another at a slight spacing, typically in the region of between approximately 3 mm and 8 mm. During operation of the lamp, the electrode tips reach temperatures of 2500xc2x0 C. and above. The discharge vessel surrounds an ionizable gas filling. The main component of this gas filling is mercury. Moreover, the filling can further comprise one or more inert gases, for example xenon, krypton or argon. Gas impurities within the discharge vessel such as oxygen, water vapour and carbon monoxide lead to oxidization and/or carbide formation at the hot electrodes. These oxides or carbides vaporize at the high electrode temperatures and are deposited on the substantially colder discharge vessel wall. This blackening of the vessel wall leads in the final analysis to an unacceptable reduction in the illumination level on the exposure substrates, for example wafers, etc.
In order to suppress the gas reactions which act negatively on the light flux and on the maintenance of the lamp, or at least to reduce them conspicuously, it is known to fasten inside the discharge vessel on the electrode lateral surfaces or on the electrode rods a getter which absorbs the gas impurities just describedxe2x80x94see, for example, U.S. Pat. No. 3,621,322. Typical getter materials in lamps of the generic type are tantalum (Ta) or zirconium (Zr)xe2x80x94see, for example, EP-A-0 715 339. The getter is fastened around an electrode rod as a metal strip or filament. The tantalum getter reaches temperatures in the region of approximately 1000xc2x0 C. to 1700xc2x0 C. and more during operation of the lamp.
However, starting the lamp constitutes a risk to the service life of the getter. It can sometimes occur in this case that the arc attaches itself to the getter before it jumps onto the electrode tips. In this case, the getter becomes instantaneously and at least locally so hot that it fuses and partially vaporizes. There is then an immediate blackening of the lamp bulb, since the vaporized getter material is deposited on the cold lamp bulb. The lamp is thereafter unusable, as a rule. For this reason, tantalum has previously been preferred as getter material, since it has a comparatively very high melting point at 2996xc2x0 C.
Longer service lives and better maintenance, that is to say as little worsening as possible of the lamp specifications such as light flux, colour rendition etc., during the service life of mercury short-arc lamps are of great importance, particularly concerning the use in exposure machines for IC fabrication, since changing a lamp means a loss of production, and substantial costs are caused thereby.
It is the object of the present invention to provide a mercury short-arc lamp in accordance with the preamble of Claim 1 having an improved maintenance.
One aspect is to provide a lamp of the generic type which has an improved getter action as regards gas impurities inside the discharge vessel.
This object is achieved in the case of a lamp having the features of the preamble of Claim 1 by the features of the characterizing part of Claim 1. Particularly advantageous refinements are to be found in the dependent claims.
It has emerged that niobium is particularly well suited as getter material under the operating conditions inside the discharge vessel of lamps of the generic type. Approximately double the reactivity of tantalum was found for niobium when oxygen and carbon dioxide were introduced as gases. This was determined by measuring the increase in mass of the respective getters before and after introducing the gas. The reactivity increases similarly to tantalum with rising temperature. However, starting from approximately 1700xc2x0 C. the vaporization of the niobium oxides produced begins, as a result of which niobium can be used sensibly as getter only below 1700xc2x0 C.
Another problem is the melting point of the niobium which, at 2468xc2x0 C., is lower than that of tantalum. Specifically, if the arc attaches itself to the niobium getter during the starting phase, there is an increased risk of local fusing of the niobium associated with a blackening of the wall of the discharge vessel.
The basic idea of the invention is to use niobium as getter material despite these problems, but to arrange the niobium in such a way as to ensure reliable protection against random arc attachment. According to the invention, for this purpose the diameter D of the electrode head is selected to be at least 1.8 times, in particular 2.5 times, better 3 times, as large as the diameter d of the electrode rod, that is to say the lamp according to the invention fulfills the condition D greater than 1.8xc2x7d, in particular D greater than 2.5xc2x7d, better D greater than 3xc2x7d. Moreover, the niobium getter is placed on the electrode rod in such a way that the further condition xcex1xe2x89xa720xc2x0, better xcex1xe2x89xa725xc2x0, is likewise fulfilled. Here, xcex1 denotes an angle whichxe2x80x94viewed in a plane containing the electrode longitudinal axisxe2x80x94is defined by the longitudinal axis of the electrode and an imaginary connecting line. This imaginary connecting line connects the end of the getter averted from the electrode head to a point on the imaginary perpendicular, running through the end of the electrode rod on the electrode head side, to the electrode longitudinal axis. The point mentioned corresponds to the projection of the maximum radius of the lateral surface of the electrode head onto this perpendicular.
What is decisive in these considerations is the overall extent of the getter, in particular the end averted from the electrode head. This end of the getter is naturally at most risk from arc attachment. That is to say, in other words it is ensured in the way according to the invention as explained above that even during the starting phase the entire getter, arranged behind the electrode head, is outside the danger zone as regards an arc attachment. By contrast, the concrete shape of the electrode head plays a subordinate role here, however. Thus, the electrode head can have an essentially circular cylindrical shape, for example, it also being possible for the getter-side edge of the electrode head to be bevelled or rounded. Finally, the electrode head can also have a non-cylindrical shape.
The niobium getter can be physically fitted to the electrode rod in multifarious forms, for example in the form of a foil or a wire filament. The getter need not necessarily be designed with edges or, seen in longitudinal section, with corners or the like for the definition of the imaginary connecting line mentioned above. The connecting line is to be understood in this regard as generalized to the extent thatxe2x80x94starting from the point obtained by projecting the maximum radius of the electrode head onto the perpendicular mentionedxe2x80x94it precisely just includes the point on the getter which is furthest removed radially and/or axially with reference to the longitudinal axis. It is ensured in this way that the arc does not attach itself to the getter when the lamp is started. Moreover, it is possible in this way also to make use of relatively extended getters. The minimum angle required according to the invention can be realized specifically, in these cases by virtue of the fact that the ratio of the diameters of the corresponding electrode head and electrode rod is selected to be suitably large. Reference may be made to the exemplary embodiments for further details on this point.
The niobium getter is preferably arranged behind the anode, since in the case of lamps conceived for DC operation, the anode is generally of more massive design than the cathode, as a result of which it is easier to fulfill the above-named conditions for reliable protection against random arc attachment.
Nevertheless, the invention is not restricted to lamps for DC operation. Rather, the invention is also to be understood as generalized to lamps with symmetrical electrodes. To this extent, the use of the terms cathode and anode are to be understood, if appropriate, as referring to the temporary function of both electrodes.
The fastening of the niobium getter can be performed by welding, soldering or mechanical joining, for example latching.
Zirconium (Zr) was initially also considered as getter material. However, it emerged very quickly that all mounting sites considered according to the invention entail an unacceptably high zirconium vapour pressure. Because of the relatively low melting point of zirconium, it is evident that no mounting site with a sufficiently low operating temperature can be found for the getter.